The present invention relates to a flat solid discharging device which may be installed in an electrophotographic copier, facsimile apparatus and other electrostatic recording apparatuses to serve as a charger or a discharger as desired.
A current trend in the electrostatic recording art is toward the use of a flat solid discharging device which may replace the traditional hairline type corona discharger. A flat solid ischarging device includse a first electrode and a second electorde which is constituted by a single flat conductive member. In the case that such a discharging device is installed in, for example, an electrophotographic copier to serve as a charger, it naturally has to uniformly charge the surface of a photoconductive element before the latter is exposed imagewise.
A problem with a prior art flat solid discharging device is that the discharge by the device is not even in the lengthwise direction, i.e., the charge deposited on teh photoconductive element is not uniform due to irregular capacitance distribution between the first and second electrodes as well as to non-uniform end configurations and material of the first electrode. Especially, when the ac voltage applied across the first and second electrodes is approximately as low as a discharge start voltage, it is difficult for the discharge to occur uniformly and, therefore, for the photoconductive element to be uniformly charged. While uniform discharge maybe achieved by increasing the ac voltage applied across the first and second electrodes, such would produce a prohovitive amount of ozone to damage the electrodes and/or to deteriorate the dielectric surfaces.
In the event when the photoconductive element is charged before exposure, should it be needlessly charged in its area other than an expected paper sheet area, toner particles not transferred to a paper sheet would remain on the photoconductive element to invite waste of toner, or developer, and an increase in the load of cleaning the remaining toner. In the light of this, various methods have heretofore been proposed for changing the charging range of a discharging device in such a manner as to match it to a particular paper sheet size. One of such methods consists in dividing the second electrode into a plurality of segments according to the sizes of paper sheets, and applying an ac voltage on a segment basis. Another method known in the art consists in using a plurality of first electrodes each of which is associated with a different paper sheet size, and selectively connecting the first electrodes to a power source based on a charging area on the photoconductive element. A drawback particular to the first-mentioned method is that the second electrodes have to be provided in a stepwise configuration in order to eliminate irregular charging otherwise caused at their junctions and, in addition, uniform charging is unattainable unless a plurality of first electrodes are used. The second-mentioned method, too, has a shortcoming that a plurality of first electrodes have to be used. Moreover, neither the first method nor the second method gives consideration to the load which acts on the power source at the instant of changeover of the electrodes. Specifically, when the electrodes are simply selectively connected to the power source, the discharge condition is changed to prevent a predetermined charge potential from being provided on the photoconductive element.